Investigation of helix-shaped and transverse crack propagation in rotor shafts based on disk shrunk technology

It is known from international feedback that the rotor shafts of the turbo-generators with disk shrunk technology may have transverse cracks located near the keys which maintain the bond between the core of the shaft and the surrounding disks in case of over speed. It was understood that the cracks were initiated by fretting between the keys and the shaft and that they propagated due to a fatigue mechanism generated by the rotational flexion of the shafts under gravity. The destructive observation now correlated to the service history of the shaft shows different mixed modes propagation phases and a stopped circumferential crack evolution during the last months of service of the shaft. Mechanical studies based on the determination of the stress intensity factors provide the evolution of the stress intensity factors during the crack propagation. They give access to information not available otherwise to explain the observed crack profiles. Finally, experimental investigations are needed to obtain the kinetics as a function of the stress intensity factors. The information provided is helpful in determining the possible crack profiles to be detected by the most suitable vibratory surveillance systems before failure in service of the shaft line.